A standard interface (I/F) between an integrated circuit of a disk controller (hereinafter referred to as disk controller IC) and an integrated circuit of a disk driver (hereinafter referred to as disk driver IC) for use in a magnetic recording system such as a floppy disk drive (FDD) and a hard disk drive (HDD) has a circuit arrangement as shown in FIG. 1.
As shown in FIG. 1, a typical disk controller IC 31 uses a second supply voltage V2 of 5 V given by the difference in potential of a third power supply of potential E3 (=5V) and a second power supply (ground) of potential Egnd. An N type MOSFET Q1 is connected between the I/F output terminal P1 of the disk controller IC and the ground, making an open drain type output transistor. A block 33 represents a built-in circuit, other than the output transistor in the IC 31, which is driven by the second supply voltage V2 (5 V). In actuality, however, the voltage V2 may be 3 V for example.
In the disk driver IC 32, a second supply voltage V2 (=5 V) given by the difference between a third potential E3 (5 V) of a third power supply and the potential Egnd of the second power supply (ground). The 5-Volt second supply voltage V2 is used as a standard I/F voltage even when the operating voltage of the internal circuit of disk driver IC 32 is lower (e.g. 3 V) than V2.
In what follows it is assumed for simplicity that in each of the ICs 31 and 32 a first supply voltage V1 (3 V) is established between the potential E1 (=3V) of a first power supply and the ground potential Egnd (the ground being a second power supply), and that the second supply voltage V2 (=5 V) is established between the third potential E3 (5 V) of a third power supply and the ground potential Egnd. (The first and the third power supplies will be also referred to by-the same reference codes E1 and E3, respectively, as their potentials.)
In the circuit arrangement shown in FIG. 1, the potential E3 (=5 V) of the third power supply is coupled with an I/F input terminal P2 via a pull-up resistor R1. The potential E3 is also coupled with the input terminal of a buffer B1. Resistors R2 and R3 are protective resistors for protecting the I/F circuit. The resistance of the resistor R2 is negligibly small as compared with the pull-up resistor R1. Since the resistors R2 and R3 are connected in series to the input terminal of the buffer B1, they do not affect the input voltage of the buffer B1.
The buffer B1 is an input buffer driven by the voltage between either potential of the first power supply E1 or the third power supply E3 and the ground potential Egnd depending on whether the operating voltage of a subsequent circuit connected to the buffer is the first supply voltage V1 (=3 V) or the second supply voltage V2 (=5 V). In the example shown in FIG. 1, the operating voltage of the subsequent circuit is assumed to be the second supply voltage V2. Diodes D1 and D2 are voltage surge protection diodes for causing positive/negative anomalous voltage surges, due to electrostatic charges for example, entering the input terminal P2 of the I/F to be absorbed by the third power supply E3 or the ground Egnd.
In this arrangement, the output terminal P1 and the input terminal P2 are connected. The N type MOSFET Q1 is turned ON/OFF in accordance with control signal SIG. The input of the buffer B1 will be pulled down to a low potential level (or the ground potential Egnd) as the N type MOSFET Q1 is turned on, while the buffer B1 will be pulled up to a high potential (or the third potential E3) as N type MOSFET Q1 is turned off.
Thus, irrespective of the supply voltage of the IC 31 being 5 V or 3 V, the voltage input to the IC 32 is converted by the I/F to the operating voltage of the internal circuit of the disk driver. The operating voltage is output from the buffer B1. It is noted that-the buffer B1 often has a hysteresis characteristic.
A conventional I/F circuit as mentioned above has a drawback, however, when it is used with a recent ½-inch height floppy disk drive (FDD) for example which utilizes a disk controller having a standard push-pull type output circuit. In this case, the IC 32 is connected to a push-pull type I/F circuit of a disk controller IC 41, as shown in FIG. 2. The output voltage of such push-pull type IC 41 is mostly 3 V.
FIG. 2 shows an exemplary I/F structure established between the disk controller IC 41 having a CMOS type 3-V output circuit and a disk driver IC 42.
As shown in FIG. 2, the IC 41 is provided with a first supply voltage V1 of 3 V. The CMOS type output circuit comprises a serially connected P type MOSFET Q2 and N type MOSFET Q1 coupled between a first power supply having a first potential E1 and a second power supply (normally the ground) of potential Egnd. It is often the case that the IC 42 of a disk driver is a conventional IC 32 as shown in FIG. 1.
The MOSFET Q1 and MOSFET Q2 are turned ON/OFF in response to a control signal SIG applied thereto. When the N type MOSFET Q1 is ON and the P type MOSFET Q2 is OFF, conditions of the two ICs are similar to that shown in FIG. 1 and no problem exists. However, when the N type MOSFET Q1 is turned off and P type MOSFET Q2 is turned on, the potential of the I/F output terminal P1, and hence the potential of the input terminal of the buffer B1, is substantially the same as the first supply voltage E1, which is about 3 V.
In this case, a current flows from the third power supply E3 of the disk driver to the first power supply E1 of the, disk controller via the pull-up resistor R1 and the protective resistor R2. The current contributes to unnecessary Joule heat in the pull-up resistor R1. On the other hand, when the P type MOSFET Q2 is turned on and the power supply of the IC 42 is cut off, power is disadvantageously supplied from the IC 41 to the IC 42 via the pull-up resistor R1 or the diode D1.